Slab and plate cooling method for producing grain oriented electrical steel

ABSTRACT

1. IN A METHOD FOR PRODUCING A HIGH MAGNETIC FLUX DENSITY, GRAIN ORIENTED ELECTRIC STEEL OR STRIP OF THE TYPE WHEREIN A STEEL INGOT PRODUCED BY CONVENTIONAL METHODS IS BROKEN DOWN INTO A SLAB, THE SLAB IS HOT ROLLED TO FORM A PLATE, THE PLATE IS COLD ROLLED IN AT LEAST ONE STEP INCLUDING A FINAL COLD ROLLING AT A REDUCTION RATE BETWEEN 65 TO 95% TO FORM A SHEET AND THE SHEET IS DECARBONIZED AND FINALLY ANNEALEED AT A TEMPERATURE ABOVE 800*C., THE IMPROVEMENT WHEREIN THE STEEL INTO CONSISTS ESSENTIALLY OF NOT MORE THAN 4.0% SI, NOT MORE THAN 0.085% CARBON 0.010 TO 0.065% ACID SOLUBLE AL, AND NOT MORE THAN 0.012% N, THE SLAB IS HEATED TO ABOVE 1200*C. TO DISSOLVE THE AIN AND THEN HOT ROLLED SUCH THAT THE SLAB IS COOLED TO A TEMPERATURE BETWEEN 1000 TO 1250*C. IN LESS THAN 200 SECONDS FROM THE TIME THE SLAB IS REMOVED FROM THE HEATING STEP, AND THE RESULTING PLATE IS COOLED FROM 1000-1250*C. TO 600*C. IN LESS THAN 200 SECONDS.

, Nov. 5, 1974 Filed Oct.

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AKIRA SAKAKURA ETAL 3,846,187 SLAB AND PLATE COOLING METHOD FORPRODUCING GRAIN ORIENTED ELECTRICAL STEEL 20, 1972 6 Sheets-Sheet 1Temperature Held for I200C x I50 sec.

Hel'd for \l 0 0C x 50 sec.

Time (sec.)

I FIG. I

Nov. 5, 1974 AKlRA SAKAKURA ETAL 3,845,187

SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICALSTEEL Filed Oct. 20, 1972 6 Sheets-Sheet 2..

Be ("b/m2) 2.3% Si' l l L85 unable for secondary recrystallization b IOO 2 00 (seconds) Holding Time at Various Temperatures FIG.2

Temperature 0) Nov. 5, 1974 AKIRA SAKAKURA ETAL 3,846.,187

SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICALSTEEL s Sheets-Sheet s Filed Oct. 20, 1972 e (Wb/m Extrusion v (Slobthickness 40 m/m) 2.95 /a Si 9 i200- IOOO' L95 Cooling Curve 600 DuringHot Rolling Water Cooling Cooled 1 I85 50() "1 Air Time from Finishingof Hot Rolling to Start of Cooling (Seconds) FIG. 5 g

Nov. 5, 1974 AK|RA SAKAKURA ETAL 3,846,187

SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICALSTEEL 6 Sheets-Sheet t Filed Oct. 20, 1972 vdE 09 On 0- m o c u ll l cul' eo i 6 .-v&%\\no\\\o l 1 1 Q mvo ON o mm o O- Om Om 007 ON m2: @500owl o Q A 9m o 2 00m 00v 00m 00m 009 OON OOE (1%) emuuadwa Nov. 5, 1974AKIRA SAKAKURA ETA!- 3.345.187

SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICALSTEEL .6 Sheets-Sheet 5 Filed 001:. 20, 1972 and 9 A35 25 s 8.: @ii 8mmd YQQE

Nov. 5, 1974 AKIRA SAKAKURA F-TAL 5. 87

SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICALSTEEL 6 Sheets-Sheet 6 Filed Oct. 20, 1972 86 $252; 33 E: w m

3,846,187 SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTEDELECTRICAL STEEL Akita Sakakura, Fumio Matsumoto, Kiyoshi Ueno, KunihideTakashima, and Katuro Kuroki, Kitakyushu, gapan, assignors to NipponSteel Corporation, Tokyo,

apan

Filed Oct. 20, 1972, Ser. No. 299,308 Claims priority, applicationJapan, Oct. 22, 1971, 46/ 83,736 Int. Cl. H01f 1/04 US. Cl. 148-112 4Claims ABSTRACT OF THE DISCLOSURE A method for producing a high magneticflux density grain oriented electrical steel sheet, comprising breakingdown and hot rolling a steel ingot containing not more than 4.0% of Si,not more than 0.0085% of C, 0.010- 0.065% of acid soluble Al, and notmore than 0.012% of N, cold rolling the plate by at least one stepincluding a final cold rolling with 65-95% reduction rate depending onthe silicon content and an intermediate annealing, decarbonizing thesheet and finally annealing the sheet at a temperature above 800 C.,said hot rolling comprising heating the slab at a temperature above 1200C. to dissolve AlN and rolling the slab into a desired plate thicknessunder the following conditions of heating cycle (1) from slab extractionfrom the heating step, and during hot rolling to produce a plate, thetime for cooling the plate to a temperature between 1000-1250 C.depending the silicon content is less than 200 seconds; and

(2) after the above cooling during hot rolling to 1000 to 1250" C., thetime for cooling the plate to 600 C., is less than 200 seconds.

The present invention relates to a process for producing so-called grainoriented electrical steel sheets having an easy magnetization in therolling direction of the steel sheet.

A grain oriented electrical steel sheet is commonly used as softmagnetic material primarily for the iron cores of electric appliances,such as, transformers, and it is important, with reference to themagnetic properties to have a good magnetization characteristic as wellas a good iron loss characteristicv Recently the minimization of size ofthe electric appliances has been increasingly important, and for thispurpose, it is necessary to reduce the weight of the iron cores. Ingeneral, in order to reduce the iron core weight, it is necessary toutilize high magnetic flux density so that magnetic materials havinggood magnetization characteristic, namely the B characteristic (magneticflux density at magnetization of 8 AT/cm.) are required (AT meansampere-turn). Also when the high magnetic flux density is utilized, theiron loss value also increases. In this connection, as compared with amagnetic material having low B characteristic, a magnetic materialhaving high B characteristic shows much better iron loss at hi hlymagnetic field and shows low increasing rate of the iron lossaccompanying the increase of the magnetic flux density.

In View of the above requirements, improvements of the magnetic fluxdensity which are necessarily required with an increased size ofelectric appliances will be realized only by developments of highmagnetic flux density grain oriented electrical steel sheets.

Further, as a means for reducing the iron loss, it is necessary toobtain a thin sheet product in order to reduce the eddy current losswhich occupies a large part of the iron loss.

"United States Patent Patented Nov. 5, 1974 Conventionally, the grainoriented electrical steel sheet was directed to products having 05-030mm. thickness, but in recent years, products having a plate thickness of12 mil (0.30 mm.) 11 ml, 9 mil and 6 mil have been increasingly used forthe iron cores of electric equipment and appliances.

The production of such a thin product can not be at tained stably by theconventional two-step cold rolling, and the production technics havebeen developed, for example, by utilizing the sulphur or seleniumpermeation process as disclosed in Japanese Patent Publication Sho43-5966. However, the thin products produced by this process showremarkably increased hysteresis loss by the thickness reduction becausethe B characteristic is about 1.8 wb./m. so that the reducing effect onthe eddy current loss is killed and thus a remarkable improvement of theiron loss can not be obtained.

On the other hand, the present inventors have developed high magneticflux density grain oriented electrical steel sheet from steelscontaining a small amount of acidsoluble Al (hereinafter called Al), andbased on this development the present inventors have further developed,through improvements of grain orientation, low-iron-loss products havingremarkably high B characteristic as high as more than 1.9 wb./rn. andshowing a very little increase in hysteresis loss even in a thin productby effectively utilizing the balance of AlN.

One of the objects of the present invention is to provide a highmagnetic flux density grain oriented electrical steel sheet which shows,as compared with the conventional electromagnetic steel sheet,remarkably excellent magnetization characteristic in the rollingdirection, namely B characteristic showing at least 1.88 Wb./II1.

Another object of the present invention is to produce stably a grainoriented electrical steel sheet having the high B characteristic asmentioned before as well as low iron loss value.

Still another object of the present invention is to produce a grainoriented electrical steel sheet which shows high B characteristic andlow iron loss value even in a thin product.

Other objects of the present invention will be clear from the followingdescription, examples and attached drawings.

FIG. 1 shows the cooling curve during the hot rolling operation, FIG. 2is a graph showing the holding temperature and time prior to thefinishing rolling in the hot rolling, and the magnetic characteristicsof the products, FIG. 3 is a graph showing the starting temperature ofrapid cooling after the completion of the hot rolling and the magneticcharacteristics of the products, FIG. 4 is a graph showing the coolingcurve during the hot rolling and the precipitation of aluminum nitride,and FIG. 5, FIG. 6 and FIG. 7 show the macro-structure and the magneticcharacteristic of the products according to Examples 4, 5 and 6respectively.

The present invention will be explained in details.

The starting material used in the present invention is an ordinary steelor silicon steel which may be made by any known steel making process andcasting method, but the chemical composition of the starting materialshould satisfy the following condition.

'c0.0ss% by weight Si4.0% by weight Al 0.0l00.065% by weight (A1 meansacid-soluble Al which will be referred to simply as Al hereinafter)Balance Fe and unavoidable impurities For example, the steel materialdisclosed in Japanese Patent Publications Sho 40-15644 and Sho 46-23820may be used as the starting material in the present invention.

Explanations will be made on the contribution by the impurities comingduring the production of the grain oriented electrical steel sheet aswell as on the eifects of Al.

Generally, in the production of grain oriented electrical steel sheets,secondary recrystallization of the socalled cube on edge having {ll} 001orientation takes place in the final annealing so that products havingexcellent magnetic characteristics in the rolling direction can beobtained. In this case the precipitates produced by the impurities suchas nitrides, sulfides and oxides play an important role.

Conventionally it has been considered that these precipitates aredispersed in fine particles and precipitate into the matrix to preventthe grain growth of the matrix. The present inventors have found thatsome of the precipitates which precipitate in a special orientationalrelation to the matrix have also ability of selectively bringing up onlythe grains having special orientation so that the orientation of thesecondary recrystallization is closely controlled, and thus productshaving excellent B characteristic can be obtained.

AlN formed by the addition of Al in the present invention is aprecipitate of the latter type, and the present invention is based onthe formation of such AlN, and it is supposed that precipitates formedby the addition of other elements do not show such ability but merelyprevent the growth of the primary recrystallization grains of thematrix.

As above explained, for the production of the grain oriented electricalsteel sheet, the presence of precipitate forming elements is absolutelynecessary, and it is very important for the final characteristics of theproducts to precipitate these elements in effective size anddistribution. In the present invention, the efiective size of theprecipitates which can contribute to the growth of the secondaryrecrystallization grains is roughly estimated to be less than 0.1g.

On the other hand, when the formation process of these precipitates inconnection with the production process of the grain oriented electricalsteel sheets, it is necessary that these precipitates of the effectivesize have been already produced in the cold matrix before the final coldrolling. Therefore, the precipitate forming process include thesolidifying step from the molten metal, the cooling step at the time ofthe break-down rolling, the cooling step during the hot rolling, and theannealing and cooling step of the hot rolled plate or intermediate thickplate before the final cold rolling. However, in the solidifying stepbefore the hot rolling or in the cooling step of the breaking downrolling, the mass of the material is so large that the cooling rate isrelatively slow and the size of most of the precipitates formed duringthese steps is larger than the eifective size. Therefore, the presentinventors have found that a grain oriented electrical steel sheet havingexcellent magnetic characteristics can be obtained by reheating thesteel slab in the hot rolling step to redissolve the precipitates againinto the matrix so as to obtain a hot rolled steel sheet havingprecipitates of eifective size by appropriate cooling at the time ofrolling, and if necessary, by subjecting the steel sheet to heattreatment before final cold rolling to give the desired precipitatecondition of the impurities to the material.

As explained above, the role played by the hot rolling step is mostimportant in the production of the grain oriented electrical steelsheet. Therefore, the present inventors have completed the presentinvention through various experiments on hot rolling using variousmaterials satisfying the required condition of the chemical composition.

A silicon steel containing 0.034% of Al and 2.3% of silicon was used asthe starting material, and three specimens of 40 mm. thick small piecewere prepared therefrom and held at 1300 C. for minutes to completelydissolve AlN into the matrix and left in the outdoor to temperatures of1200 C., 1100 C. and 1000 C. respectively, and immediately held for50-250 seconds in furnaces held at 1200 C., 1100 C. and 1000 C.respectively, then hot rolled to 3.2 thickness by two passes, and cooledin the air. The thus hot rolled plates were cold rolled into products of0.35 mm. thickness. The relation between the B characteristic of theproducts, the holding temperature and the holding time before the hotrolling is shown in FIG. 2. One example of the cooling curve of thematerial during the hot rolling is shown in FIG. 1 (A) in FIG. 1 showsthe cooling curve obtained when the rolling was conducted immediatelyafter the slab extraction. (B) and (C) show cooling curves obtainedrespectively by holding the material at 1100 C. for 50 seconds and at1200 C. for 150 seconds.

As clearly understood from FIG. 2, in case of holding temperature of1000 C., the characteristic is deteriorated by a 50 second holding, andwhen the holding time is more than seconds, the secondaryrecrystallization itself is unstable. In case of holding times of 1100C. and 1200 C., similar tendencies appear as in case of 1000 C., but ahigher holding temperature tends to increase the holding time allowablefor the characteristic deterioration and the appearance of secondaryrecrystallization.

However, if the material is held at 1100 C. for 200 seconds, thesecondary recrystallization is prevented from taking place and held at1200 C. for more than 200 seconds, the magnetic property deteriorates.This phenomenon may be attributed to the assumption that most of AlNwhich has been dissolved into the matrix by the holding at 1300 C. for30 minutes precipitates during the holding between 1000 and 1200 C. sothat the amount of AlN of effective size precipitating during thecooling in the hot rolling step becomes relatively small, and the latterprecipitation progresses rapidly at a relatively low-temperature holdingas at 1000 C., while it progresses slowly at a high-temperature holding.

Similar tendencies are observed by the results of similar experiments asin FIG. 2 made on materials having different silicon contents in respectto the holding temperature and time. In this case, however, theallowable temperature and time ranges vary in correspondence to thesilicon contents. Namely, in case of 1.0% silicon content, thedeterioration of the characteristics is observed as the holding timebecomes longer in case of the holding temperature of 1000 C., but theoccurence of the secondary recrystallization grains is stable even withthe holding time of seconds. In the case of a holding temperature of1100 C., some deterioration of the characteristic is observed as theholding time become longer, but the occurence of the secondaryrecrystallization grains is stable even when the holding time exceeds200 seconds.

When the holding time is at 1200 C., the holding time has no influenceon the deterioration of the characteristic and on the occurence of thesecondary recrystallization grains in case of a 1.0% silicon content.This indicates that the temperature of 1200 C. itself is satisfactoryfor the solid dissolution of AlN in the slab. On the other hand, whenthe silicon content in the material is increased, the allowable holdingtemperature and time before the rolling becomes extremely narrow. Forexample, in case of 3.15% silicon content, the occurence of thesecondary recrystallization grains becomes unstable regardless theholding time, and a holding'temperature of 1150 C. at the minimum isnecessary and the holding time is also desirably within 50 seconds inorder to avoid the deterioration of the characteristic.

FIG. 4 shows the results of observations of the relation between thecooling cycles of hot rolling and the amount of AlN precipitate bychanging the silicon content. In case of 2.8% silicon content, AlNstarts to precipitate about 1250 C. and the precipitation progressesrapidly below 1200 C., while in case of 1.1% silicon content, AlN doesnot substantially precipitate at a temperature down to 1000 C., andstarts to precipitate at a temperature below 1000 C. This is consideredto be due to the fact that the oc'y transformation zone of the materialincreases or decreases in correspondence to its carbon and siliconcontents, and the behaviour of AlN precipitation is related closely tothe amount of 7 phase.

Therefore, the phenomenon that the slow cooling zone before the hotrolling changes in accordance with the silicon content is also quitereasonable from the results in FIG. 4. Also careful consideration shouldbe given to the precipitation of AlN after the completion of the hotroll ing. FIG. 3 shows the relation between B characteristic and thestarting temperature of water quenching in case when the product wasobtained by heating 3% Si steel at 1350 C. for 30 minutes andimmediately rolling it to a finished thickness of 3.5 mm., and waterquenching the plate from the temperature immediately after the hotrolling, and finally subjecting the hot rolled plate to the productionprocess for a grain oriented electrical steel sheet. It is understoodfrom the results that better product characteristics are obtained whenthe material is cooled rapidly from the possible earliest stage afterthe hot rolling to 600 C. at which most of AlN precipitate is completed,namely, cooled as rapidly as possible wiihin a range below 200 seconds.The effect of the silicon content of the material at this stage issimilar to that on the slow cooling zone beofre the hot rolling and at ahigher silicon content, it is necessary to rapidly cool the materialfrom a higher temperature zone. At a low silicon content, the desiredcharacteristics are obtained even by slow cooling from a relatively lowtemperature zone, and it is understood that the amount of the a-vtransformation of the material is relevant in this stage also. i

As for the cooling cycle in the rot rolling of grain oriented electricalsteel sheet containing a very small amount of Al, the cycle as shown inFIG. 1 (A) is most desirable. Namely the heating of the slab before thehot rolling should be done at a temperature and time sufficient forredissolving fully the AlN into the matrix and the hot rolling should bedone by holding the material temperature after the slab extraction up tothe starting of the rolling (finish rolling) at a highest temperaturefor the shortest time possible, and it is necessary that the material iscooled as rapidly as possible to room temperature immediately after thecompletion of the rolling.

In the case of hot rolled steel plates obtained by the above mostdesirable cooling cycle, the high temperature heat treatment for AlNredissolution and recrystallization may be omitted and yet excellentproduct characteristics can be obtained as shown in Example (1).

From the above, the hot rolling conditions for production of a grainoriented electrical steel sheet having excellent directional propertiesutilizing the etfect of AlN for preventing grain growth maybe defined asfollows. After the slab material is heated to a temperature above 1200C. in accordance with the silicon content to dissolve AlN into thematrix, the heat cycle in rolling the slab to a desired plate thicknessshould satisfy the following condition.

(l) The time after the slab extraction until the material is cooled to atemperature of 1000-1250" C. depending on the silicon content is notmore than 200 seconds.

(2) After the above cooling to 1000-1250 C., the time for cooling theplate to 600 C. is not more than 200 seconds.

As explained before, when the steel sheet is treated according to thepresent invention, a secondary recrystallization structur of very highorientation is caused by Al (more strictly AlN), but in a certain rangea higher B characteristic namely {1l0} 100 secondary recrystallizationstructure having higher accumulation degree can be obtained as the Alcontent increases. Thus, when such a material is used to produce a finalproduct of thickness more than about 0.35 mm. (14 mil), very excellentcharacteristics can be obtained stably. However, in case of theproduction of a final product of thickness less than 0.3 mm. (12 mil),the product is more susceptible to the 6 influence of the other elementssuch as C, Si and N and production conditions as the Al contentincreases, and thus unless these factors are closely controlled, theamount, size and distribution of AlN becomes unbalanced to causeincomplete secondary recrystallization.

In order to correct such conditions, it is necessary to keep AlNeffectively in good balance, thereby it is possible to increasestability in the chemical composition and the production conditions andto expand the allowable range of Al to a higher Al content so thatstable final products having remarkably improved characteristics,particularly thin products of thickness less than 0.3 mm. can beobtained.

Namely, in the production of high magnetic flux density grain orientedelectrical steel sheet from Al-containing steels, the present inventorshave succeeded to commercially produce final products, particularly thinfinal products having more perfect secondary recrystallization and veryexcellent characteristics by nitricling the hot rolled material asmentioned before in a continuous annealing treatment.

The reasons why the final products having further improvedcharacteristics can be obtained by an appropriate nitriding may beconsidered as follows. Although the magnetic properties become veryexcellent as the Al content increases, but the secondaryrecrystallization becomes slightly unstable due to the unbalance of theAlN content.

As disclosed in the Japanese Patent Publication Sho 46-23820, theprecipitated AlN of specific fine size is present in a specific amount.AlN has such ability that the grain growth of the matrix along thegrowth of the secondary recrystallization nuclei is prevented, but onlythe grains having a specific orientation relation to the precipitatingdirection of the fine AlN are selectively allowed to grow, thus theorientation of the secondary recrystallization is closely controlled sothat very sensitive l00 orientation can be obtained.

When the AlN content is high, it is favourable to the {l00} 100orientation and the selection of the secondary recrystallization grainsbecause of a large amount of the specific AlN, but the growth of thesecondary recrystallization nuclei is prevented by the strong preventiveeffects due to the great amount of AlN, thus causing incompletesecondary recrystallization.

When the nitriding treatment is applied, the enriched nitrogen reducesthe amount of fine AlN into an appropriate amount and eliminates thefactor preventing the growth of the specific secondary recrystallizationnuclei so that the secondary recrystallization is made stable.

Also, nitrogen combines with Al in the steel and makes it possible toprovide an appropriate amount of the specific fine AlN as necessityarises. Thus, even if the Al content is high, the selectivity of 100secondary recrystallization grains can be maintained and their growth iseffectively controlled.

In the nitriding treatment which is done to facilitate the formation ofthe effective AlN, the required amount of enriched nitrogen variesbetween 0.0005 and 0.004% depending on the chemical composition of thematerial, particularly the A1 content and the N content, the workinghistory before the nitriding treatment and the thickness of the finalproduct, and in this way it is desirable that the total amount ofnitrogen in the steel plate is 0.005- 0.012%. After the amount ofnitrogen has been adjusted to the above range, the precipitationtreatment which forms the effective AlN is applied to form 0.0005 to0.0095% of AlN.

If the amount of enriched nitrogen is out of the above range, forexample below the lower limit, the secondary recrystallization becomesunstable, and above the upper limit, the very high B characteristicwhich is the feature of a high magnetic flux density steel sheet can notbe obtained any more, and the stability of the secondaryrecrystalization becomes poor.

From the aspect of enriching nitrogen, it may be considered to add allnecessary amounts of nitrogen during the steel making, but it is desiredthat the components, particularly the Al content is increased in orderto obtain the thin products of thickness less than 0.30 mm. (12 mil),having very excellent characteristics, and thus the amount of nitrogenrequired for the formation of effective AlN is naturally increased.However, if a large amount of nitrogen is added during the steel making,blisters are more apt to occur in the final product, thus lowering theyield of the final product.

On the other hand, if the nitriding is done during the continuousannealing of the hot rolled steel plate, such an undesirable phenomenondoes not take place even if the nitrogen content increases. Also thenitriding during the continuous annealing of the hot rolled plate isvery advantageous int hat close control can be done to the material inwhich the amount and formation of effective AlN is unbalanced due tovariations of the components (Al, N) in the steel making and variationsof hot rolling conditions.

The nitriding can also be done in the continuous decarburizationannealing of the cold rolled steel sheet of final thickness, but in thiscase it is very difficult to control closely the nitrization amount, andthus the production is unstable although excellent characteristics areobtained in some cases.

Any nitriding source may be used for the nitrization, and for example,gas containing nitrogen compounds such as NH and NO may be added to thefurnace gas, or a nitrogen compound may be coated directly to the steelsheet. The use of N gas as nitrogen source is not efiicient because Ngas is inert. In any way, activated nitrogen is supplied and enrichedfor the nitrization.

The nitrogen enrichment to the steel sheet by the nitrogen compound gas,such as, NH and NO is achieved by introducing these nitrogen compoundgases either or mixed with the furnace gas into an annealing furnace andsupplying them to the steel sheet. In the case when the nitrogenenrichment is done by coating a nitrogen compound onto the steel sheet,the compound is applied before the annealing in front of the furnace ina similar way as the separator is applied before the final annealing.For example, powders of manganese nitride and so on are mixed with waterand dropped to the coating roll while stirring and coated on the surfaceof the steel sheet. The amount of nitrogen to be given is adjusted bythe coating amount.

In case of NH gas which is most commonly used,

the amount of active nitrogen source required for the before mentionedrange of enriched nitrogen must be more than 0.2% expressed in themixing proportion to the furnace gas, although it depends on the gasflow rate and time.

The nitriding treatment is desirably done in the continuous annealingstep of the hot rolled steel plate, but it may be done before this stepif required. In this case, the nitriding is effected at a temperatureabove 600 C. for 30 seconds to 30 minutes, and the amount of nitridingsource to be supplied must be more than 0.2% in the mixing proportion tothe furnace gas in case of NH After the nitriding treatment, the steelplate is cooled once to the room temperature and successively subjectedto the ordinary continuous annealing, or after the nitriding treatmentthe steel plate is successively subjected to the ordinary continuousannealing and rapidly cooled to cause effective AlN precipitates.

The material used in the present invention, as mentioned before, is anordinary steel or a silicon steel containing less than 40% of siliconand 0.0l-0.065% of aluminum and in the form of steel ingots obtained bythe conventional steel making and melting method, or in the form ofsteel slabs obtained by the continuous casting or the pressure casting.Commercially produced steel slabs contain more than 0.0020% of nitrogen,which is well enough for the formation of AlN important to the presentinvention. The above materials are hot rolled to 1.5-7 mm. thickness,after the break-down rolling to destruct the cast structure, ifnecessary. The AlN precipitation annealing after the hot rolling butbefore the final cold rolling is as fully disclosed in the JapanesePatent Publication Sho 46-23820, and its key points are that thematerial is annealed in the temperature range of 7501200 C. for 30seconds to 30 minutes and is cooled, and in this cooling step, thematerial should be rapidly cooled through the temperature range from 950to 400 C. in 2 to 200 seconds. The cold rolling in the present inventionis done in such a way that the final cold rolling is done at a reductionrate of -95% depending on the silicon content. The decarburizationannealing and the final annealing after the cold rolling may be done bya conventional method.

EXAMPLE 1 A silicon steel ingot containing 0.050% of C, 3.05% of Si,0.030% of Al and 0.028% of S was broken down to a slab thickness of 40mm., which was held at 1350 C. for 30 minutes and immediately subjectedto a finish rolling. The starting temperature of the rolling was 1280 C.and the time after the slab extraction to the start of the rolling was15 seconds. The slab was reduced to a thickness of 3.5 mm. by two passesin the finish rolling, the finishing temperature of the rolling was 1120C. and the time required after the slab extraction up to the completionof the rolling was 35 seconds. Then the plate was rapidly cooled inwater at 20 C. The time required by the rapid cooling was 10 seconds.

The thus obtained hot rolled plate was acid pickled, cold rolled by to afinal thickness of 0.35 mm., subjected to a continuous decarburizationannealing and a final annealing in H at 1200 C. for 20 hours. Themagnetic characteristics in the rolling direction of the product were asfollows:

EXAMPLE 2 A silicon steel ingot containing 0.045% of C, 2.3% of Si,0.025% of Al and 0.013% of S was broken down to a slab thickness of 40mm., which was held at 1300 C. for 30 minutes and immediately held in afurnace at 1200 C. for seconds, and then subjected to the finishrolling. The time required after the slab extraction to the start of thefinish rolling was seconds, and the finish rolling was done by twopasses to obtain a plate thickness of 4.0 mm. at a finishing temperatureof 1050 C. The time after the slab extraction to the completion of therolling was 210 seconds. The hot rolled steel plate thus finished wasimmediately subjected to a rapid cooling in water to room temperature.The time required by the water cooling was about 10 seconds. The hotrolled steel plate was then acid pickled, cold rolled by 25%, annealedin an N atmosphere at 1100 C. for 2 minutes, and quenched in hot waterat 100 C. Then the plate was acid pickled, cold rolled by 88% to a finalthickness of 0.35 mm., continuously decarburized, and subjected to thefinal annealing in N at 1200" C. for 20 hours. The magneticcharacteristics in the rolling direction of the final product were asfollows:

B,=1.9s3 (Wb./m.

W 17/50=1.32 (W./kg.)

The relative low iron loss as compared with the B characteristic is dueto the unusually large grain size of the product.

EXAMPLE 3 A silicon steel ingot containing 0.053% of C, 2.80% of S1,0.032% of Al, and 0.027% of S was broken down to a slab of thickness of40 mm., which was held at 1350 C. for 30 minutes and immediatelysubjected to a finish rolling.

The starting temperature of the rolling Was 1280 C., the time requiredafter the slab extraction up to the start of the rolling was 15 seconds.The slab was reduced to a plate thickness of 2.8 mm. by three passes inthe finish rolling at a finishing temperature 980 C. The time requiredafter the slab extraction up to the completion of the rolling was 45seconds. The cooling after the rolling was done in the air and the timerequired for cooling the plate to 300 C. was 180 seconds. The hot rolledplate was then annealed in N atmosphere at 1150 C. for 2 minutes, andcooled from 1150 C. to the room temperature in 45 seconds by usingvapour-water spray. The annealed plate was acid pickled, cold rolled toa final thickness of 0.30 mm., subjected to a decarburization annealingand a final annealing in H at 1200 C. for 20 hours. The magneticcharacteristics in the rolling direction of the product were as follows:

B =1.923 (-wb./m.

W 17/50=1.05 (W./kg.)

EXAMPLE 4 The steel ingots A and B having the chemical compositionsshown in Table 1 were broken down, hot rolled and subjected to thefollowing treatments shown hereunder. The processes up to the hotrolling were same as in Example 3.

Cold rolling Continuous decarburizatton annealing (850 0.)

Finish annealing (1200 C.)

Table 2 shows the increment AN of nitrogen after the nitridingtreatment, the total nitrogen amount TN and the amount of precipitatedAlN (N as AlN).

FIG. 5 shows the macro-structure and magnetic characteristics of theproduct obtained by the above process.

In case of the product of 0.35 mm. thickness excellent characteristicswere obtained regardless of the nitriding treatment. On the other hand,in the case of the product of 0.30 mm. thickness, the secondaryrecrystallization is incomplete and the characteristics are poor whenthe nitriding treatment is not applied. When the nitriding treatment wasapplied, the secondary recrystallization was complete and excellentcharacteristics were obtained. Thus it is clear that the effect of thenitriding treatment is remarkable in the case of thin products.

The steel ingot A used in Example 4 was broken down,

hot rolled under similar conditions as in Example 4 and subjected to theprocesses as shown hereunder.

Hot rolled plate (1) No coating Coating of nitride (2) Manganese nitride(N fgrigeongl was coated Continuous annealing (1100 C. 2 minutes N2100%) Acid pickling Cold rolling (0.23 mm.)

Continuous decarburization annealing (850 0.)

Finish annealing (1200 C.)

Table 3 shows the increment AN of nitrogen after the nitridingtreatment, the total nitrogen TN and the precipitated AlN.

FIG. 6 shows the macro-structure and the magnetic characteristics of theproduct obtained by the above processes.

TABLE 3 Process (2) AN 0.0023 TN 0.0079 AlN (N as AlN) 0.0063

EXAMPLE 6 The steel ingot A used in Example 4 was broken down, hotrolled under similar conditions as in Example 4 and subjected to thefollowing processes.

esses. It is understood the effect of the nitriding treatment isremarkable.

TABLE 4 Process (2) AN 0.0030 TN 0.0086 AlN (N as AlN) 0.0069

What is claimed:

1. In a method for producing a high magnetic flux density, grainoriented electric steel sheet or strip of the type wherein a steel ingotproduced by conventional methods is broken down into a slab, the slab ishot rolled to form a plate, the plate is cold rolled in at least onestep including a final cold rolling at a reduction rate between 65 to toform a sheet and the sheet is decarbonized and finally annealed at atemperature above 800 C., the improvement wherein the steel intoconsists essentially of not more than 4.0% Si, not more than 0.085%carbon, 0.010 to 0.065%. acid soluble Al, and not more than 0.012% N,the slab is heated to above 1200 C. to diSr solve the AlN and then hotrolled such that the slab is cooled to a temperature between 1000 to1250 C. in less than 200 seconds from the time the slab is removed fromthe heating step, and the resulting plate is cooled from 1000-1250" C.to 600 C. in less than 200 seconds.

2. A method according to claim 1 in which the hot rolled plate issubjected to a high temperature continuous annealing in a temperaturerange of 750-1200 C. for 0.5 to 30 minutes and rapid cooling toprecipitate AlN before the final cold rolling.

3. A method according to claim 2 in which the nitrogen is enriched by0.0005-0.004% in the plate while the plate is subjected to thecontinuous annealing between 750-1200 C. by contacting the plate with anitrogen-containing compound and the annealed plate is rapidly cooledfrom the annealing temperature range of 750-950 C. to 400 C. in 2-200seconds to obtain 0.0005-0.0085% of precipitated AlN.

4. A method according to claim 2 in which prior to the continuousannealing, the hot rolled plate is treated at a temperature above 600 C.for 30 seconds to 30 minutes by contacting the plate with anitrogen-containing pound to enrich the nitrogen content in the plate by0.0005-0.004%, and the subsequent continuous annealing is etfected at atemperature of 750-1200 C. for 30 seconds-30 minutes, and the plate israpidly cooled from 12 a temperature range of 750-950 C. to 400 C. in2-200 seconds to obtain 0.0005-0.0095% of precipitated AlN in the plate.

References Cited UNITED STATES PATENTS 3,671,337 6/1972 Kumai et al.148---12l 3,620,856 11/1971 Hiraoka l48121 3,180,767 4/1965 Easton etal. 148l2 3,165,428 1/1965 Albert et al 148-111 3,632,456 1/1972Sakakura et al 148l12 3,636,579 1/1972 Sakakura et al. 148-112 FOREIGNPATENTS 726,154 1/1966 Canada 148-111 WALTER R. SA'ITERFIELD, PrimaryExaminer US. Cl. X.R.

1. IN A METHOD FOR PRODUCING A HIGH MAGNETIC FLUX DENSITY, GRAINORIENTED ELECTRIC STEEL OR STRIP OF THE TYPE WHEREIN A STEEL INGOTPRODUCED BY CONVENTIONAL METHODS IS BROKEN DOWN INTO A SLAB, THE SLAB ISHOT ROLLED TO FORM A PLATE, THE PLATE IS COLD ROLLED IN AT LEAST ONESTEP INCLUDING A FINAL COLD ROLLING AT A REDUCTION RATE BETWEEN 65 TO95% TO FORM A SHEET AND THE SHEET IS DECARBONIZED AND FINALLY ANNEALEEDAT A TEMPERATURE ABOVE 800*C., THE IMPROVEMENT WHEREIN THE STEEL INTOCONSISTS ESSENTIALLY OF NOT MORE THAN 4.0% SI, NOT MORE THAN 0.085%CARBON 0.010 TO 0.065% ACID SOLUBLE AL, AND NOT MORE THAN 0.012% N, THESLAB IS HEATED TO ABOVE 1200*C. TO DISSOLVE THE AIN AND THEN HOT ROLLEDSUCH THAT THE SLAB IS COOLED TO A TEMPERATURE BETWEEN 1000 TO 1250*C. INLESS THAN 200 SECONDS FROM THE TIME THE SLAB IS REMOVED FROM THE HEATINGSTEP, AND THE RESULTING PLATE IS COOLED FROM 1000-1250*C. TO 600*C. INLESS THAN 200 SECONDS.